This invention relates to the formation of silicon-bearing material, particularly to the formation of coatings by thermal decomposition, and reaction with oxygen, of silicon-bearing reactants carried in the vapor phase to a hot surface on which the coating is deposited.
The most pertinent published art is disclosed in U.S. Pat. Nos. 4,187,336 and 4,206,252 and the teaching of these patents are incorporated by reference herein. In general, they relate to the formation of transparent silicon-oxide bearing coatings by the oxidation of silanes, sometimes in combination with other materials such as tin compounds to obtain coatings of a particular refractive index. Such coatings have application, for example, as anti-iridescent interlayers on a glass substrate beneath sub-micron-thick coatings of infra-red-reflective materials such as fluorine-doped tin oxide.
In the published art, the silanes which are utilized included such materials as: EQU SiH.sub.4,(CH.sub.3).sub.2 SiH.sub.2,(C.sub.2 H.sub.5).sub.2 SiH.sub.2 and (CH.sub.3).sub.4 Si.sub.2 H.sub.2.
These materials provided highly satisfactory coatings. However, they had flammability characteristics and oxygen sensitivity that required special consideration in storage, handling and use. Silane (SiH.sub.4) is spontaneously flammable in air, and while the other silanes listed are not spontaneously flammable, they do absorb and react with oxygen from the air, on standing. In the use of those silanes for chemical vapor deposition of films, their high flammability can lead to uncontrolled reactions and undesirable formation of powder, unless reaction conditions are carefully controlled. Therefore, it was decided to search for less flammable substitute materials which would still provide suitable silicon-bearing coatings. Despite the decreased oxidation susceptibility of any substitute reactant compounds, it would be necessary for them to permit achievement of favorable reaction at rates which are suitable for continuous coating processes, e.g. coating rates which are practical for use on float glass production lines.
Simultaneous oxidation of silanes and a tin-containing compound, such as tetramethyltin, produces mixed silicon-tin oxide coatings which are not spatially uniform in composition or refractive index. This differential reactivity can be applied to advantage, as in U.S. Pat. No. 4,206,252, to produce anti-iridescent gradient coatings. However, in other applications, as in U.S. Pat. No. 4,187,336, homogeneous layers of more uniform composition and refractive index are desirable. It has proven difficult to find a silicon material whose reactivity closely matches that of tetramethyltin, and would thus oxidize simultaneously with tetramethytin to produce coatings of homogeneous refractive index.